Molecular imaging with targeted imaging probes can be used to evaluate vascular phenotype in cardiovascular disease. In the past funding period, we developed new tools for assessing endothelial activation and cellular adhesion using contrast-enhanced ultrasound (CEU) molecular imaging in order to detect and quantify high risk atherosclerotic disease. Because CEU molecular imaging can detect events that occur at the endothelial-blood pool interface with high sensitivity and temporal resolution, it is a potentially valuable asset in preclinical research than can be used to discover new disease-related pathways or to rapidly test new candidate therapies. The overall aim of this proposal is to leverage the technologies we have developed in order to better understand the role of platelets throughout the course of atherosclerotic plaque development. Platelet-endothelial interactions have been postulated to play an important early role in promoting atherosclerosis by potentiating the inflammatory response. These interactions have not been well characterized because of lack of appropriate tools for assessing platelet- endothelial interactions in vivo. In Aim 1 CEU molecular imaging of platelet GPIb? and endothelial activation in a murine model of atherosclerosis will be used to temporally characterize platelet-endothelial interaction and endothelial inflammatory activation at different stages of plaque development. Mechanism for platelet adhesion will also be investigated by: (a) imaging the effects of inhibitors to the molecular pathways that may mediate platelet-endothelial or platelet-monocyte complexes, and (b) direct molecular imaging of potential mediators such as dysregulated von Willebrand factor. Since increased oxidative stress is a likely common mediator of endothelial activation and platelet adhesion, in Aim 2, we will determine whether platelet-endothelial interactions at various stages of atherosclerosis are modifiable by potent anti-oxidant therapies (NADPH-oxidase inhibitors, catalase mimetics, or stable transfection with single chain antibodies against oxidized lipoproteins). CEU molecular imaging will be an important biologic readout for these studies. In Aim 3, we will evaluate endothelial-platelet interactions and endothelial inflammatory activation in primates by studying obese, insulin- resistant, and atherosclerotic rhesus macaques. In these studies CEU molecular imaging will be combined with targeted radionuclide imaging of vascular lipid oxidation. Treatment effects of NADPH oxidase inhibitors will again be used to determine the regulatory role of oxidative stress. Successful completion of these studies will provide important new information on atherosclerosis pathophysiology which can be targeted for therapy.